Sources of variation between two serial tests must be considered in interpreting if there was a clinically significant change. The main causes of variation are the biological variation coefficient (CVB) of the test in question, which must be obtained from the literature, and the analytical coefficient of variation (CVA) of the same test, which must be obtained from the internal quality control laboratory data. With both data we can calculate the critical difference or reference change value which helps us to decide whether there was a real change in the patients health.